ABSTRACT: The 20 October, 2005 Georgian Bay earthquake: a postglacial rebound event?

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Abstract

On October 20 2005 at 21:16 UTC, a small earthquake (Mn = 4.3) occurred in Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67ą N, 80.46ą W). Despite its small magnitude, this earthquake was exceptionally well recorded by the POLARIS seismograph network, and is of particular interest due to its location 90 km from a proposed long-term storage facility for high-level nuclear waste. No damage was reported, but the earthquake produced an audible noise and was felt to a distance of 100 km with a maximum intensity of IV MM. Within 24 hours of the main shock, four portable seismograph systems were installed in the epicentral region to record aftershocks. The main shock was preceded by 30 s by a foreshock, and was followed by 5 recorded aftershocks within a 4-day period. All 7 epicenters in this earthquake sequence cluster within a 1.25 km radius. The extensive high-quality seismograph recordings have enabled a comprehensive analysis of this event. Pg and Sg arrival times at epicentral distances from 27 to 282 km were inverted to obtain a crustal velocity model, with a Moho at 40 km depth and a linear velocity increase in the crust from 6.2 to 7.0 km/s. A clearly visible depth phase (sPg) is indicative of a mid-crustal focal depth (10.5 § 0.5 km), in marked contrast to the shallower seismicity that characterizes Lake Ontario and Lake Erie earthquakes. After correction for attenuation, average displacement, velocity and acceleration spectra in the range 0.4 - 5.0 Hz Żt a simple Brune model with seismic moment M0 = 3.8 § 1.0 Ł 1014 N-m and corner frequency 3.0 § 0.4 Hz. Moment tensor inversion of body waves using a double couple mechanism indicates a short-duration source time function(ź 0.05 s rise time) and reverse-sense of slip, with a steeply dipping NW-striking nodal plane and a shallow-dipping N-striking nodal plane. The NW-striking nodal plane is the most probable fault plane, since this is more consistent with the aftershock distribution. Structural fabric of the (inaccessible) crust in this area can be determined from aeromagnetic maps, which show a strong NW-striking lineation. This pattern is probably caused by maŻc dykes, belonging to either the 2.45 Ga Matachewan swarm or 0.7 Ga Grenville swarm. We interpret the inferred reverse motion on a steeply dipping fault plane as a crustal response to regional postglacial isostatic adjustment by slip along a pre-existing zone of weakness associated with dyke emplacement. The discrepancy between the rise time obtained by waveform modeling (ź 0.05 s) and the rise time inferred from the spectral corner frequency (fcĄ1 = 0.33 s), coupled with the observation of a small foreshock, are suggestive of self-healing pulses of slip along this fault.

Bibliography of Canadian Geomorphology